1. Field of the Invention
The present invention relates to an optical system for an optical head used in an optical disc system and, more particularly, to a focus error detecting element for generating a focus error signal by producing an astigmatism for focusing an objective lens on an optical disc at all times.
2. Description of the Prior Art
In the optical disc system, a condensed spot of light formed by an objective lens must be focused on an information carrier surface at all times. To achieve this, it is necessary to detect any deviation of the condensed light spot from the information carrier surface so that an error if detected can be fed back to accomplish a servo control. The prior art head optical system used in the optical disc system is shown in FIG. 5.
Referring to FIG. 5 for the discussion of the prior art, rays of light emitted from a semiconductor laser 25 are converted by a collimator lens 26 into a parallel bundle of light which is subsequently converged by an objective lens 27 on the information carrier surface 29 formed on a rear surface of an optical disc substrate 28. The light modulated by the information carrier surface 29 is reflected therefrom so as to travel towards a beam splitter 30 through the objective lens 27. As the reflected light passes through the objective lens 27, it is transformed into a parallel returning beam. The beam splitter 30 deflects the returning beam so as to travel towards a photodetector 34 through a convex lens 31, then through a cylindrical lens 32 and finally through a concave lens 33. Change in distance between the objective lens 27 and the information carrier surface 29 results in a defocusing. Accordingly, once the defocusing occurs, the modulated returning beam reflected from the information carrier surface 29 does not become a parallel bundle of light after having passed through the objective lens 27, and may diverge or converge outwardly. On the other hand, an astigmatism occurs on the photodetector 34 due to the presence of the cylindrical lens 32. If the photodetector 34 is positioned intermediate of the astigmatic difference, a substantially round light spot is formed on the photodetector 34 when in an in-focus condition. However, if the light incident on the photodetector 34 is diverged or converged depending on a focus error, the spot profile on the photodetector 34 varies with the astigmatism caused by the cylindrical lens 32. By detecting a change in pattern of light distribution by means of the quadrant photodetector 34, it is possible to obtain a focus error signal. (See "Principles of Optical Disc System" by G. Bouwhuis, et al., Adam Hilger Ltd., 1985, Chap. 2, p. 79.)
Assuming that the focal length of the objective lens 27 is f.sub.ob and the composite focal length of an assembly of the convex and concave lenses 31 and 33 is f.sub.dt, the sensitivity of a focus error detecting optical system is proportional to f.sub.ob.sup.2 /(f.sub.dt.sup.2 .multidot..DELTA.f), wherein .DELTA.f represents the spacing between two focal points produced by a cylindrical surface of the cylindrical lens 32. On the other hand, the size of the light spot formed on the quadrant photodetector 24 is proportional to f.sub.dt /.DELTA.f. Accordingly, since .DELTA.t is determined if the necessary sensitivity is secured, the size of the light spot is proportional to the focal length f.sub.dt of the detecting optical system. If the size of the light spot falling on the quadrant photodetector 34 is too small, not only is it difficult to properly position the photodetector 34 relative to the light spot, but also the width of a gap between each neighboring photodetectors can no longer be negligible. If therefore the focal length f.sub.dt of the detecting optical system is chosen to be of a great value, the size of the optical system will become large making it difficult to render the optical head to be compact. The concave lens 33 is employed for eliminating the foregoing problems since it forms a part of a telephoto optical system wherein a concave and a convex lenses are employed with the convex lens located on one side of the concave lens adjacent a light source and which generally has a short optical system for a focal length.
However, with the system shown in FIG. 5, since the three lenses are necessary for the focus error detecting optical system, assemblage requires a precise adjustment, making the optical system expensive. Also, even though this is replaced with a toric surface, the concave lens is still required for the optical system to become compact and, hence, two lenses, i.e., toric and concave lenses, are required.